Vehicle systems can include a fuel cell stack that generates energy to power a propulsion system and/or auxiliary electrical systems over a voltage bus. The propulsion system includes an electric machine that generates drive torque. Excess energy generated by the fuel cell stack can be stored in an energy storage device (ESD). Energy generated by other means including, but not limited to, regenerative braking can also be stored by the ESD. The ESD can be used to selectively supplement the fuel cell stack to power the propulsion and/or the auxiliary electrical systems. For example, the ESD can be used to power vehicle system components during a start-up period.
During normal operation, power flows from the fuel cell stack to the propulsion system. Under certain operating conditions, the propulsion system operates in six-step mode. In six-step mode, the torque capability of the electric machine is maximized at higher speeds. When operating in the six-step mode, however, a low frequency ripple current is generated in the voltage bus. The ripple current can adversely affect the fuel cell stack.
In order to adequately filter the low frequency ripple current from the voltage bus, a relatively large capacitance is required. In the case of film type bulk capacitors, it is not feasible to produce a sufficiently large capacitor. Although aluminum electrolytic capacitors can be used, they have relatively low current handling capability. Therefore, a significant number of capacitors would be required, which adversely effects overall system cost and size.